Use Serpent cleaning putty around all the holes and wires on the receiver and ESC. This will keep the water out of the electronics and is easily removed. I ran in standing water at the last HPI World in Halifax England with only Serpent putty to keep the water out of the electronics and never had a problem.
Spray all the metal pieces and bearings lightly with WD40. This will repell the water so it won't puddle up and rust.
Most importantly take a hair dryer and dry off the car after each run to avoid rust.
Water won't short your battery since it DC and waters resitance is in the megaohms.
I didn't bother with bagging the motor. As long as its not submerged a running motor will likely blow any water right out of itself.
In the picture the grey blob is KO 302F receiver under a mound of Serpent putty.
Hope this helps.